Tunnel diodes, where carriers tunnel through the band gap of a doped p-n junction, have taken many forms since first proposed in about 1958. Tunnel diodes provide very fast switching time and low power dissipation. The first tunnel diode, called an Esaki tunneling diode after the originator, comprised two silicon regions of different conductivity types with both being highly doped. Present day Esaki type diodes may comprise germanium, gallium arsenide, or other semiconductor material. When bias is applied to the Esaki type diode, the available states for electrons in the contact layer align with available states for holes in the valence band of the injection layer and tunneling occurs. This Esaki type diodes have low peak current densities, low peak-to-valley current ratios, and low operational frequencies.
The conventional double quantum well heterostructure interband diode comprises two doped layers sandwiched between a contact layer and an injection layer similar to the Esaki diode, but with two quantum wells, separated by a tunneling barrier layer, between the two doped layers. When sufficient bias is applied to approximately align the ground state for holes in one quantum well with the ground state for electrons in the other quantum well, tunneling occurs. Then an electron from the filled ground state can transfer to the collector layer and an electron from the injection layer can replace the transferred electron. As further bias is applied, tunneling current decreases because electron and hole states are no longer aligned. Valley current includes leakage and thermionic currents that degrade performance. Valley current increases proportionally with the bias. The band gap of the interband tunneling barrier layer substantially blocks the thermionic current which contributes to the valley current.
A high current density is also desired, but it tends to reduce peak-to-valley current ratio. Current density is current for a given area, usually given as amps per centimeter squared.
Although much progress has been made, resonant tunnel diodes have traditionally had a high peak current density with a low peak-to-valley current ratio. Heterostructure interband tunneling diodes have traditionally had the reverse, a high peak-to-valley current ratio with a lower peak current density.
Therefore, a heterostructure interband tunneling diode having a simplified structure that is easily manufactured, and having an increased current density while preserving the peak to valley current ratio is needed.